In the technical field of photon detection, the counting and detection of individual photons is assuming an ever-increasing importance.
In molecular biology, for example, the detection of molecules is performed using fluorescence or luminescence phenomena, frequently characterized by extremely reduced light emission.
Extremely sensitive sensors are moreover required in the digital capture of three-dimensional images in reduced lighting conditions.
To this end, Geiger-mode avalanche photodiodes find a wide range of uses in so far as their high quantum efficiency allows detection of both individual photons and packets of photons.
Unlike a PN-junction photodiode, a Geiger-mode avalanche photodiode operates at a reverse biasing voltage that is higher than the breakdown voltage.
The sensitivity to incident photons and, hence, the likelihood of detection depend upon the reverse biasing voltage of the photodiode. In particular, the greater the reverse biasing voltage, beyond the breakdown threshold, the greater the likelihood of an avalanche generation of charge carriers occurring.
In this condition and in the absence of incident photons, an individual charge carrier generated in conditions of darkness, for example by transfer of thermal energy, is sufficient to trigger the process of avalanche carrier generation by impact ionization, generating a flow of current referred to as “dark current”.
The dark current is an undesirable effect in this type of devices in so far as it generates an electrical signal even in the absence of incident photons and may adversely interfere with the normal use of the device.
In addition, Geiger-mode avalanche photodiodes belonging to an array of photodiodes are extremely sensitive not only to the photons that impinge thereon, but also to charge carriers (for example, electrons) generated by the adjacent photodiodes sharing the same substrate and to photons generated by electroluminescence during the avalanche multiplication in adjacent photodiodes. These effects are known, respectively, as “electrical cross-talk” and “optical cross-talk”.
In order to exploit fully the sensitivity of the Geiger photodiode for detection of individual photons and to reduce the negative effect of electrical and optical cross-talk, the active regions of said photodiodes are typically made such that the crystal lattice has an extremely small number of defects. In this condition, a carrier generated in dark conditions statistically traverses a long mean free path before generating an avalanche effect through an impact-ionization mechanism.
The above solution does not, however, reduce the sensitivity of the array. In fact, in the time interval corresponding to the free path, which is relatively long, of a charge carrier, an incident photon may generate an electron-hole pair, which triggers the process of avalanche generation, thus causing a flow of current associated with the incident photon, which enables detection thereof.
Since the process of avalanche generation is self-sustaining, it is moreover necessary to implement a circuit for quenching the avalanche effect and resetting the photodiode so as to render it available for detection of a further photon. Currently known quenching circuits are of two types: active ones and passive ones.
In the passive-quenching mode, a resistor having a high resistance is set in series to the photodiode. A photon impinging upon the photodiode determines an increase of current in the photodiode and in the series-connected resistor, causing a voltage drop that reduces the electrical field that sustains the avalanche carrier generation to a value lower than that of the breakdown voltage. Consequently, the avalanche carrier generation is interrupted.
In the active-quenching mode, a purposely designed external circuit detects the increase of current caused by an impinging photon and reduces the voltage on the photodiode below the breakdown threshold using a switch that couples the photodiode to a resistor having a high resistance and operating analogously to the described passive mode. In both the passive and the active modes, at the end of the photodiode-current resetting, the reverse voltage applied thereto again reaches a high value, higher than the breakdown voltage.
Getting back to the problem of optical and/or electrical cross-talk, some solutions have been proposed.
According to a first solution proposed for the reduction of optical cross-talk, each photodiode is insulated from adjacent photodiodes by metal trenches having the function of mirroring the photons responsible for optical cross-talk; this solution is typically ineffective as regards electrical cross-talk.
According to a second solution, aimed at eliminating both optical and electrical cross-talk, each photodiode will be insulated from the adjacent photodiodes by V-shaped grooves. This approach, however, drastically reduces the possibility of high integration of the components on account of the considerable area occupied by the V-shaped grooves.
According to a third solution, which is also suitable for eliminating both types of cross-talk, the individual photodiodes are provided in separate dies assembled mechanically within a same package. In this solution, the photodiodes do not share their own substrate with the adjacent photodiodes, and a total insulation is obtained both from the optical and the electrical standpoint. Also the latter solution has the disadvantage of not enabling a high level of integration of the components and has high production costs.